They did as much if not more work than their husbands or partners

Taking the second part on its own:- "They did more work as their husbands ..." is incorrect.

"Than" cannot be replaced by "as". It's just that an "as" is missing.
It should be:- "They did as much as, if not more work than their husbands or partners".

It would better rewritten as:-
"They did at least as much work as their husbands ..."

DC